Search Results (76)

Mechanical unloading of skeletal muscle triggers various signaling alterations that result in muscle atrophy and weakness. Mitochondria are essential to muscle health, acting not only as energy suppliers but also as central mediators of molecular regulation. Mitochondrial activity, content, and dynamics are tightly controlled by multiple signaling pathways; conversely, mitochondria-derived messengers, such as reactive oxygen species (ROS), ATP, and mitokines, are involved in the regulation of nearly all aspects of muscle signaling. During mechanical unloading, altered muscle activity leads to mitochondrial dysfunction. However, the initial triggers, underlying mechanisms, and full consequences of this dysfunction remain poorly understood. Nevertheless, mitochondria-targeted therapies have emerged as a promising strategy for mitigating unloading-induced muscle impairments. In this review, we summarize current data regarding the characteristics, causes, and outcomes of unloading-induced mitochondrial dysfunction, specifically focusing on muscle atrophy and functional decline. We highlight novel findings regarding the roles of mitokines and mitochondrial calcium overload, propose a new hypothesis to explain the biphasic dynamics of ATP accumulation during slow-type muscle unloading, and describe emerging therapeutic strategies to counteract these mitochondrial impairments. Full article

The depolarization of the sarcolemma is one of the first effects of unloading on skeletal muscle. We hypothesized that unloading-induced activation of the dihydropyridine receptor (DHPR), a voltage-sensitive L-type Ca²⁺ channel, and depolarization of the sarcolemma trigger intracellular Ca²⁺ release from the sarcoplasmic reticulum and activation of Ca²⁺-dependent signaling pathways, resulting in muscle atrophy. Nifedipine, a DHPR calcium channel blocker, was used to study the role of DHPR in the regulation of signaling pathways during three days of rat soleus muscle unloading/hindlimb suspension. Inhibition of the DHPR during unloading attenuates the decrease in soleus muscle contractile properties, prevents the accumulation of ATP, ROS, and Ca²⁺ content in the sarcoplasm and the mitochondria, and blocks the decrease in PGC1alpha mRNA expression and Junctophilin-1 (JP1) proteolysis. In nifedipine-treated rats, the improvement of the unloaded soleus muscle contractile properties could be mediated by blocking the calpain-mediated degradation of the cytoskeletal proteins. DHPR blocking could be one of the future directions for the preservation of contractile properties of inactive/unloaded muscle. Full article

Excessive alcohol consumption promotes clinical myopathy and injury-related immobilization. Because both alcohol and disuse jeopardize muscle health, their combined effects may synergistically accelerate fiber type-dependent muscle wasting. Ten-week-old male C57BL/6J mice were fed a control or 5% alcohol-diet for 3 weeks (NIAAA-model), with or without 1 week of unilateral hindlimb immobilization, generating four sets of limb muscles (n = 9/grp): control (CO), with immobilization (CI), alcohol (AL), with immobilization (AI). Gastrocnemius and soleus muscles were atrophied by CI, AL, and AI, whereas quadriceps atrophy was induced by CI and AI only (all p < 0.05). In soleus, CI, AL, and AL decreased p-mTOR (~40–60%, p < 0.01) and p-p70S6K (~50–87%, p < 0.05), indicating suppressed anabolic signaling. In contrast, in the quadriceps, alcohol increased p-4EBP1 by ~200% (p < 0.01), while p-Akt was elevated by ~180%, only in AI (p < 0.01). Myogenesis signaling was inhibited by alcohol and immobilization. For protein degradation, immobilization increased MAFbx by >50% in both muscles (p < 0.01). Quadriceps exhibited increased p-PERK (+53%) under AI (p < 0.05), whereas several markers of ER stress were reduced by all interventions in soleus (p < 0.05). These findings suggest that alcohol consumption does not exacerbate immobilization-induced atrophy; however, alcohol suppresses anabolic signaling in soleus, suggesting greater susceptibility to myopathy. Full article

Among ground-based paradigms used to reproduce altered gravity exposure, the hindlimb unloading (HU) model is widely employed to simulate microgravity conditions by removing gravitational loading from the hindlimbs. Despite its extensive use, behavioral adjustments during suspension remain poorly characterized, although they may provide valuable indicators of animal welfare and individual susceptibility. Here, we comprehensively characterized the behavioral profile of mice during and after HU using a dedicated ethogram, with the aim of identifying behavioral markers associated with individual coping strategies. Several exploratory and postural behaviors showed marked time-dependent modulation, with baseline exploratory activity predicting a more adaptive behavioral trajectory during suspension, possibly indicative of greater resilience. In parallel, brain levels of the neurotrophins NGF and BDNF were measured to explore their relationship with behavioral outcomes. Although no significant group differences were detected, suspended mice displayed a progressive reduction in both neurotrophins over time, which paralleled behavioral adaptation. Together, these findings indicate that specific exploratory behaviors represent reliable predictors of resilience to HU, while NGF and BDNF may reflect ongoing neuroplastic processes associated with prolonged suspension. Full article

Background/Objectives: Polygonatum sibiricum (PS), possessing both medicinal and edible dual functions, boasts a long history of application in Chinese traditional practices. As a component of its effectiveness, Polygonatum sibiricum polysaccharides (PSPs) have been reported to exert neuroprotective effects. However, the protective effects of PS on the cognitive deficits induced by simulated weightlessness remain unclear. This study evaluated the therapeutic potential of PSPs to counteract the cognitive deficits induced by simulated weightlessness using the Hindlimb Unloading (HU) method. Methods: Mice were subjected to HU to establish cognitive impairment, and PSP was administered for four weeks. The Morris water maze test (MWMT) and passive avoidance test (PAT) were used to evaluate the cognitive abilities of mice, followed by an analysis of molecular mechanisms. Results: PSP treatment increased learning and memory in mice. PSP treatment partially restored gut microbial diversity and composition towards beneficial taxa, including Lactobacillus and Firmicutes, while inhibiting proinflammatory genera, including Alistipes and Proteus. At the same time, PSP upregulated Claudin-5 and Zonula Occludens-1 (ZO-1) levels in the colon, suggesting improved intestinal barrier integrity, and decreased neuroinflammatory response by inhibiting NLRP3 inflammasome activation and NF-κB phosphorylation in the hippocampus. It also modulated neurotransmitter homeostasis along the microbiota–gut–brain (MGB) axis by increasing the levels of gamma-aminobutyric acid (GABA) and serotonin (5-HT) while reducing the levels of excitotoxic metabolites, including Glutamate (Glu) and 3-hydroxykynurenine (3-HK). Conclusions: These results indicate that PSP may have beneficial effects on HU-induced cognitive impairment by regulating gut microbiota, enhancing barrier function, suppressing neuroimmune signaling, and restoring neurotransmitter balance. Full article

Microgravity exposure during spaceflight has been linked to cognitive impairments, including deficits in attention, executive function, and spatial memory. Both space missions and ground-based analogs—such as head-down bed rest, dry immersion, and hindlimb unloading—consistently demonstrate that altered gravity disrupts brain structure and neural plasticity. Neuroimaging data reveal significant changes in brain morphology, functional connectivity, and cerebrospinal fluid dynamics. At the cellular level, simulated microgravity impairs synaptic plasticity, alters dendritic spine architecture, and compromises neurotransmitter release. These changes are accompanied by dysregulation of neuroendocrine signaling, decreased expression of neurotrophic factors, and activation of oxidative stress and neuroinflammatory pathways. Molecular and omics-level analyses further point to mitochondrial dysfunction and disruptions in key signaling cascades governing synaptic integrity, energy metabolism, and neuronal survival. Despite these advances, discrepancies across studies—due to differences in models, durations, and endpoints—limit mechanistic clarity and translational relevance. Human data remain scarce, emphasizing the need for standardized, longitudinal, and multimodal investigations. This review provides an integrated synthesis of current evidence on the cognitive and neurobiological effects of microgravity, spanning behavioral, structural, cellular, and molecular domains. By identifying consistent patterns and unresolved questions, we highlight critical targets for future research and the development of effective neuroprotective strategies for long-duration space missions. Full article

Spaceflight and microgravity (μg) environments induce numerous cardiovascular changes that affect cardiac structure and function, and understanding these effects is essential for astronaut health and tissue engineering in space. This review compiles and analyzes over 30 years of research on the impact of real and simulated μg on cardiomyocytes. A comprehensive literature search was conducted across five databases, and 62 eligible studies involving cardiac cells under μg or spaceflight conditions were compiled and analyzed. Despite the great heterogeneity in terms of cardiac model, microgravity platform, and exposure duration, multiple studies consistently reported alterations in Ca²⁺ handling, metabolism, contractility, and gene expression. Three-dimensional human-induced pluripotent stem cell-derived cardiomyocyte (HiPSC-CM) models generally showed enhanced tissue maturation and proliferation parameters, suggesting potential therapeutic benefits, while 2D models mostly exhibited stress-related dysfunction. In vivo simulated microgravity studies, such as the hindlimb unloading (HU) model, show structural and functional cardiac remodeling, and real μg studies confirmed various effects seen under the HU model in multiple rodent species. Thus, μg exposure consistently induces cardiac changes at the cellular and molecular level, while model choice, microgravity platform, and exposure duration critically influence the outcomes. Full article

Objective: Prolonged microgravity environments impair skeletal muscle homeostasis by triggering fiber-type transitions and metabolic dysregulation. Although exercise and nutritional interventions may alleviate disuse atrophy, their synergistic effects under microgravity conditions remain poorly characterized. This study investigated the effects of an 8-week ketogenic diet combined with aerobic exercise in hindlimb-unloaded mice on muscle fiber remodeling and metabolic adaptation. Methods: Seven-week-old male C57BL/6J mice were randomly divided into six groups: normal diet control (NC), normal diet with hindlimb unloading (NH), normal diet with hindlimb unloading and exercise (NHE), ketogenic diet control (KC), ketogenic diet with hindlimb unloading (KH), and ketogenic diet with hindlimb unloading and exercise (KHE). During the last two weeks of intervention, hindlimb unloading was applied to simulate microgravity. Aerobic exercise groups performed moderate-intensity treadmill running (12 m/min, 60 min/day, and 6 days/week) for 8 weeks. Body weight, blood ketone, and glucose levels were measured weekly. Post-intervention assessments included the respiratory exchange ratio (RER), exhaustive exercise performance tests, and biochemical analyses of blood metabolic parameters. The skeletal muscle fiber-type composition was evaluated via immunofluorescence staining, lipid deposition was assessed using Oil Red O staining, glycogen content was analyzed by Periodic Acid–Schiff (PAS) staining, and gene expression was quantified using quantitative real-time PCR (RT-qPCR). Results: Hindlimb unloading significantly decreased body weight, induced muscle atrophy, and reduced exercise endurance in mice. However, the combination of KD and aerobic exercise significantly attenuated these adverse effects, as evidenced by increased proportions of oxidative muscle fibers (MyHC-I) and decreased proportions of glycolytic fibers (MyHC-IIb). Additionally, this combined intervention upregulated the expression of lipid metabolism-associated genes, including CPT-1b, HADH, PGC-1α, and FGF21, enhancing lipid metabolism and ketone utilization. These metabolic adaptations corresponded with improved exercise performance, demonstrated by the increased time to exhaustion in the KHE group compared to other hindlimb unloading groups. Conclusions: The combination of a ketogenic diet and aerobic exercise effectively ameliorates simulated microgravity-induced skeletal muscle atrophy and endurance impairment, primarily by promoting a fiber-type transition from MyHC-IIb to MyHC-I and enhancing lipid metabolism gene expression (CPT-1b, HADH, and PGC-1α). These findings underscore the potential therapeutic value of combined dietary and exercise interventions for mitigating muscle atrophy under simulated microgravity conditions. Full article

Hemojuvelin (HJV) is a membrane-bound protein prominently expressed in the skeletal muscle, heart, and liver. Despite its established function in iron regulation, the specific role of HJV in muscle physiology and pathophysiology is not well understood. In this study, we explored the involvement of HJV in disuse-induced muscle atrophy and uncovered the potential mechanisms. Hindlimb unloading (HU) resulted in soleus muscle atrophy in wild type (WT) mice, accompanied by a significant decrease in HJV protein expression. The muscle-specific deletion of Hjv (MKO) exacerbated myofiber atrophy, which was associated with an increase in the expression of muscle ubiquitin ligases following HU. Furthermore, the expression of transforming growth factor-β type II receptor (TβRII) and the level of phosphorylated Smad3 (p-Smad3) were elevated after HU, and these effects were exacerbated in MKO mice. The knockdown of TβRII in the skeletal muscle of MKO mice mitigated myofiber atrophy and reversed the hyperactivation of the TβRII/Smad3 pathway induced by HU. Our findings demonstrate that the absence of HJV contributes to the activation of the TβRII/Smad3 signaling pathway and, consequently, the onset of myofiber atrophy in response to HU. Given its abundant expression in skeletal muscle, HJV emerges as a potential therapeutic target for muscle atrophy. Full article

Skeletal muscle atrophy, which is induced by factors such as disuse, spaceflight, certain medications, neurological disorders, and malnutrition, is a global health issue that lacks effective treatment. Hindlimb unloading is a commonly used model of muscle atrophy. However, the underlying mechanism of muscle atrophy induced by hindlimb unloading remains unclear, particularly from the perspective of the myocyte proteome and metabolism. We first used mass spectrometry for proteomic sequencing and untargeted metabolomics to analyze soleus muscle changes in rats with hindlimb unloading. The study found 1052 proteins and 377 metabolites (with the MS2 name) that were differentially expressed between the hindlimb unloading group and the control group. Proteins like ACTN3, MYH4, MYBPC2, and MYOZ1, typically found in fast-twitch muscles, were upregulated, along with metabolism-related proteins GLUL, GSTM4, and NDUFS4. Metabolites arachidylcarnitine and 7,8-dihydrobiopterin, as well as pathways like histidine, taurine, and hypotaurine metabolism, were linked to muscle atrophy. Protein and metabolism joint analyses revealed that some pathways, such as glutathione metabolism, ferroptosis, and lysosome pathways, were likely to be involved in soleus atrophy. In this study, we have applied integrated deep proteomic and metabolomic analyses. The upregulation of proteins that are expressed in fast-twitch fibers indicates the conversion of slow-twitch fibers to fast-twitch fibers under hindlimb unloading. In addition, some differentially abundant metabolites and pathways revealed the important role of metabolism in muscle atrophy of the soleus. As shown in the graphical abstract, our study provides insights into the pathogenesis and treatment of muscle atrophy that results from unloading by integrating proteomics and metabolomics of the soleus muscles. Full article

Background: Lutein, a carotenoid, exhibits various biological activities such as maintaining the health of the eye, skin, heart, and bone. Recently, we found that lutein has dual roles in suppressing bone resorption and promoting bone formation. In this study, we examined the effects of lutein in a disuse-induced osteoporosis model using hindlimb-unloaded (HLU) mice. Methods: Osteoclast differentiation was assessed by coculturing mouse primary osteoblasts and bone marrow cells or culturing a mouse osteoclast precursor cell line. The bone-resorbing activity was determined by mouse calvarial organ cultures. An in situ docking simulation was conducted to reveal the interaction of lutein and IκB kinase (IKK) β protein. HLU mice were fed a 1% lutein-containing diet for two weeks, and the femoral bone mass was measured by μCT. Results: Osteoclast differentiation is significantly inhibited by lutein, astaxanthin, and β-cryptoxanthin. In contrast, only lutein promoted osteoblastic calcified bone nodule formation. To elucidate the molecular role of lutein, we functionally analyzed the NF-κB complex, a molecule involved in bone metabolism, especially in osteoclasts. Docking simulations showed that lutein binds to IKK, thus inhibiting the activation of NF-κB. In a cell culture analysis, the phosphorylation of p65, the active form of NF-κB in osteoblasts, was suppressed by lutein treatment. In vivo, a μCT analysis of the bone microarchitecture showed that lutein improves several bone parameters while maintaining bone mass. Conclusions: Lutein is effective in maintaining bone mass by controlling both bone resorption and formation, which is applied to prevent disuse-induced osteoporosis. Full article

Osteoporosis is the most prevalent metabolic bone disease, especially when aggravated by aging and long-term bed rest of various causes and also when coupled with astronauts’ longer missions in space. Research on the use of static magnetic fields (SMFs) has been progressing as a noninvasive method for osteoporosis due to the complexity of the disease, the inconsistency of the effects of SMFs, and the ambiguity of the mechanism. This paper studied the effects of mice subjected to hindlimb unloading (UL, HLU) and reloading by the 0.2 T–0.4 T static magnetic field (MMF). Primary bone marrow mesenchymal stem cells (BMSCs) were extracted to explore the mechanism. Eight-week-old male C57BL/6 mice were used as an osteoporosis model by HLU for four weeks. The HLU recovery period (reloading, RL) was carried out on all FVEs and recovered in the geomagnetic field (45–64 μT, GMF) and MMF, respectively, for 12 h/d for another 4 weeks. The tibia and femur of mice were taken; also, the primary BMSCs were extracted. MMF promoted the recovery of mechanical properties after HLU, increased the number of osteoblasts, and decreased the number of adipocytes in the bone marrow. MMF decreased the total iron content and promoted the total calcium content in the tibia. In vitro experiments showed that MMF promoted the osteogenic differentiation of BMSCs and inhibited adipogenic differentiation, which is related to iron metabolism, the Wnt/β-catenin pathway, and the PPARγ pathway. MMF accelerated the improvement in bone metabolism and iron metabolism in RL mice to a certain extent, which improved the bone quality of mice. MMF mainly promoted osteogenic differentiation and reduced the adipogenic differentiation of BMSCs, which provides a reliable research direction and transformation basis for the osteoporosis of elderly, bedridden patients and astronauts. Full article

Chronic neuromuscular inactivity induces capillary regression within skeletal muscle. The objective of this study was to investigate the potential effects of dietary nucleic acids in counteracting the capillary reduction linked to chronic neuromuscular inactivity in the soleus muscle. The study utilized four distinct groups of female Wistar rats: a control group (CON), a hindlimb-unloading group (HU), an HU group supplemented with DNA (HU + DNA), and an HU group supplemented with RNA (HU + RNA). For a duration of two weeks, rats in the HU + DNA and HU + RNA groups were administered 1500 mg/kg of DNA or RNA orally on a daily basis. Two weeks of hindlimb unloading was concomitant with a reduction in the absolute weight of the soleus muscle and the capillary-to-fiber (C/F) ratio. This was associated with changes due to disuse, including increased accumulation of reactive oxygen species (ROS) and reduced levels of superoxide dismutase (SOD-2), along with elevated levels of thrombospondin-1 (TSP-1), an anti-angiogenic factor. Administering DNA at a medium concentration in the diet did not effectively prevent the reduction in the ratio between capillaries and fibers. In contrast, the equivalent concentration of RNA successfully averted the regression of capillaries during the unloading phase. Additionally, reactive oxygen species (ROS), superoxide dismutase-2 (SOD-2), and thrombospondin-1 (TSP-1) protein were kept at the same levels as in the control. The aforementioned findings reveal that RNA is more effective than DNA in preventing capillary regression triggered by muscle atrophy. Full article

In most mammals, postural soleus muscles are involved in the maintenance of the stability of the body in the gravitational field of Earth. It is well established that immediately after a laboratory rat is exposed to conditions of weightlessness (parabolic flight) or simulated microgravity (hindlimb suspension/unloading), a sharp decrease in soleus muscle electrical activity occurs. However, starting from the 3rd day of mechanical unloading, soleus muscle electrical activity begins to increase and reaches baseline levels approximately by the 14th day of hindlimb suspension. This phenomenon, observed in the course of rat hindlimb suspension, was named the “spontaneous electrical activity of postural muscle”. The present review discusses spinal mechanisms underlying the development of such spontaneous activity of rat soleus muscle and the effect of this activity on intracellular signaling in rat soleus muscle during mechanical unloading. Full article

The aim of this study was to determine whether horses exhibiting unilateral hindlimb lameness unload (rest) the lame limb more than the contralateral limb. The resting/unloading of the hindlimbs and the time spent lying down were measured using accelerometers. Ten non-lame horses and 20 lame horses were recruited for participation and monitored for 11 h overnight with accelerometers (MSR145, sampling rate: 1 Hz, and measuring range: ±15 g) attached to the lateral metatarsal and metacarpal regions of each limb. Metatarsal and metacarpal orientation were used to determine whether the limb was unloaded (rested) or loaded, respectively, or whether the horses were lying down. The relation of resting time between non-lame and lame limbs (non-lame/lame: 0.85 ± 1.2) of the lame horses differed significantly (p = 0.035) from that of the non-lame horses (right/left: 1.08 ± 0.47). Non-lame horses rested their hindlimbs evenly (left: 15 ± 10%; right: 17 ± 16%). Horses with unilateral hindlimb lameness unloaded the lame limb longer (lame limb: 61.8 ± 25.3%, non-lame limb: 38.2 ± 25.3%) than their contralateral limb. The lame horses (13 ± 11%) lay down longer (p = 0.012) than the non-lame horses (3 ± 6%). The degree of lameness determined by the participating veterinarians (Vet Score) (r = −0.691, p < 0.01) and the asymmetry evaluated by the lameness locator (ALL) (r = −0.426, p = 0.019) correlated with the resting ratio (rest time ratio). Both factors were also correlated with the time spent lying down (Vet Score (r = 0.364, p = 0.048) and the ALL (r = 0.398, p = 0.03)). The ALL and VET Score were significantly correlated (r = 0.557, p = 0.01). The results of this study provide a good baseline for future research into how individual resting patterns may help to detect pain. Full article